A motif is a detail within the story that repeats itself throughout the work. Examples of common motifs include colors, character traits, objects, locations, or situations. The sky's the limit, really. What makes something a motif is when it shows up several times throughout the story. Think of them as breadcrumbs left by the author to draw your attention toward something important in the theme or message of the story.
What is a motif, and how can I find them in Macbeth?
Before you start thinking that only classic authors like Shakespeare put motifs in their work, let's take a quick look at the movie The Sixth Sense for an example of how motifs are used in all sorts of literature. The next time you watch the movie, watch for how writer/director M. Night Shyamalan uses the color red. The character Cole wears a red sweater, a red balloon floats up toward the attic in the birthday party scene, Bruce Willis's character Malcolm jiggles a red doorknob several times, and you'll probably be able to find several more examples.
When you pick up on a repetitive detail, such as the color red in The Sixth Sense, the question to start asking yourself is: Why? Sure, it could be just a coincidence — but authors (and movie directors) are usually pretty careful and intentional about the details that they put into their stories. So, look for a repetitive detail and then think about what the author might be up to. In this movie, most people agree that the color red is a motif being used to give you clues that something supernatural is going on. The flashbacks at the very end of the movie include a lot of those same red things, which is no coincidence.
As you look for motifs in Macbeth, keep your eyes and ears open for details that seem to be intentionally repeated throughout the play, especially if those details help reveal something about the main idea of the story.